1. Field of the Invention
The present invention relates to a dispersant for dispersing carbon nanotubes and a carbon nanotube composition comprising the same. More particularly, the present invention relates to a dispersant having a rigid linear block and a flexible block with a link imparted therebetween, which is so highly adsorptive onto carbon nanotubes that it prevents the aggregation of carbon nanotubes, thereby increasing the dispersibility of the carbon nanotubes, and a carbon nanotube composition comprising the same.
2. Description of the Related Art
A carbon nanotube is honeycomb lattice rolled into a cylinder, in which one carbon atom is connected with others in a hexagonal pattern. Having a diameter on the order of a few nanometers, a carbon nanotube exhibits characteristic electrochemical properties as well as excellent mechanical properties. For example, carbon nanotubes are known to have high electrical selectivity, excellent field emission properties and highly efficient hydrogen storage properties. The structure of a nanotube strongly affects its electrical properties, that is, whether it is metallic or semiconducting, and its energy gap depends on the tube diameter. Having a quasi-one dimensional structure, in addition, a carbon nanotube exhibits characteristic quantum effects. Owing to these characteristics, interest in the fundamental properties and potential applications thereof has been rapidly growing, particularly in electronics, bio-engineering, and medicine.
For instance, carbon nanotubes find use in various fields of electroconductive films, field emission displays (FED), scanning probe microscopes (SPM), and the like, and are actively and extensively studied.
For use in the formation of electroconductive films or the fabrication of various electronic devices, carbon nanotubes need to be effectively dispersed in matrices such as solutions or binders. However, carbon nanotubes exhibit a great tendency to aggregate in bundles in a matrix owing to Van der Waals forces. When aggregated in a matrix, carbon nanotubes cannot exhibit the characteristic properties thereof and cannot be formed into a film having uniform properties throughout.
This strong tendency toward aggregation makes it difficult to sufficiently disperse carbon nanotubes in a matrix using commercially available dispersants. Extensive attempts have been made to develop novel dispersants and methods to uniformly disperse carbon nanotubes in solvents or binders.
For instance, a method of introducing an alkyl group into a carbon nanotube using a chemical linkage has been disclosed. An alkyl group having 8 or more carbon atoms can increase the solubility of carbon nanotubes in organic solvents to hundreds of parts per million (ppm), but adds an insulation value, thereby decreasing electroconductivity. One the other hand, a smaller alkyl group cannot increase solubility to the desired extent.
A method of wrapping carbon nanotubes with a polymer which is physically interactive with the nanotubes, thereby increasing the solubility thereof, has also been disclosed. However, the carbon nanotubes wrapped with the polymer are disconnected from each other, so that the electroconductivity thereof is low. Further, when the coating thereof is not perfect, the polymers and the carbon nanotubes both aggregate, leading to a decrease in dispersion.
Also disclosed is a technique wherein a functional group selected from among cyan, amine, hydroxy, carboxyl, halide, nitrate, thiocyan, thiosulfate, vinyl, and combinations thereof is attached to carbon nanotubes. This method, however, damages the surface of carbon nanotubes to the extent of degrading the electrical properties thereof.
Linear dispersants, such as sodium dodecylbenzene sulfonates (NaDDSS), sodium dodecyl sulfate (SDS), and the like, including the diblock type dispersant shown in FIG. 6a, are known to disperse carbon nanotubes. However, these dispersants are disadvantageous in that they are difficult to adsorb on the surface of carbon nanotubes owing to the linear structure thereof.
Also, triblock type dispersants, such as a,w-dioligoethyleneglycol-oligothiophene (FIG. 6b) have been introduced. Having a strong tendency toward self-aggregation to form micelles, these dispersants adsorb only weakly onto carbon nanotubes and thus show only limited effects of improving the dispersibility thereof.